1. Field of the Invention
The present invention relates to a magnetoresistive sensor, a thin film magnetic head, a wafer for thin film magnetic heads, a head gimbal assembly, a head arm assembly, a head stack assembly and a hard disk device, and particularly to a magnetoresistive sensor for a thin film magnetic head of a magnetic storage device such as a hard disk device.
2. Description of the Related Art
In order to cope with the recent trend toward high-density magnetic storage, a magnetic head has been developed using a GMR (Giant Magnetoresistive) sensor as a read sensor. In particular, a GMR sensor using SV (spin valve) configuration yields large magnetoresistive ratio for a sense current, supplied to the sensor to read records stored in the storage medium, and can provide a magnetic head of a high sensitivity. In the above description, SV configuration refers to a multi-layered structure formed of a ferromagnetic layer in which the direction of magnetization is pinned in one direction (hereinafter referred to a pinned layer also), a ferromagnetic layer in which the direction of magnetization is varied depending on an external magnetic field emanating from a storage medium (hereinafter, referred to as a free layer also) and a non-magnetic intermediate layer (hereinafter referred to as a non-magnetic spacer layer) interposed between the pinned and free layers. In the SV configuration, the direction of magnetization of the free layer makes an angle relative to the direction of magnetization of the pinned layer, depending on the external magnetic field, causing a change in spin-dependent scattering of conduction electrons as a function of the relative angle, entailing a change in magnetoresistance. The magnetic head detects this change in magnetoresistance and reads the magnetic information stored in the storage medium.
While interests to MR (magnetoresistive) sensors using SV configurations have conventionally been focused on a CIP (Current in Plane)-GMR sensor in which a sense current flows parallel to the layer surface, there has also recently, in order to cope with higher areal density, the development of the magnetic head using a CPP (Current Perpendicular to the Plane)-GMR sensor in which a sense current flows perpendicularly to the layer surface. CPP sensors include a TMR (Tunnel Magnetoresistance) sensor using a TMR layer. The CPP-GMR sensor, however, has been expected to be a sensor of high potential, because the CPP-GMR sensor has a low resistance compared to the TMR sensor, and also has a capability of providing a high output power even in scanning a narrow track compared to the CIP-GMR sensor.
However, it is impossible for the CPP-GMR sensor to attain an enough amount of change in magnetoresistance when the SV configuration in the CIP-GMR sensor is applied to the CPP-GMR sensor. This is mainly because the resistance of the layers that contribute to the change in magnetoresistance (the free layer, the pinned layer and non-magnetic spacer layer) is too small for the overall resistance of the sensor. Specifically speaking, the main reason is in that: while in a CIP sensor, the direction of the current flow is parallel to the layer surface, therefore the change in magnetoresistance caused by spin-dependent scattering can be attained enough in the parallel direction, in a CPP-GMR sensor, however, the sense current is passed through in a direction perpendicular to the layers, i.e., to the boundaries, therefore the spin-dependent scattering at the boundaries is not sufficiently caused; furthermore, a conventional GMR sensor has only two boundaries between the non-magnetic spacer layer and the free and pinned layers. Thus, the contribution of the boundaries to the change in magnetoresistance is small. In view of this problem, Specification etc. of Japanese Patent Laid-open Publication No. 2003-152239 describes a technology intended to increase the magnetoresistive effect in which a non-magnetic spacer layer is interposed into a free layer or a pinned layer to increase the number of boundaries. Specifically, a free layer and a pinned layer in which the free layer is formed of a stack of a first ferromagnetic layer of CoFeB, non-magnetic layer of Cu, and a second ferromagnetic layer of CoFeB was proposed (the CoFeB/Cu/CoFeB stacked structure). Since spin polarization at the boundary between CoFe alloy and Cu is large in the above layer structure, the spin scattering was enhanced and a large change in magnetoresistance was caused.
Furthermore, for example, Specification etc. of Japanese Patent Laid-open Publication No. 2002-359412 describes the technology of enhancing the change in magnetoresistance by sandwiching a high resistance layer in any of the free, pinned and antiferromagnetic layers, thereby causing an increase in resistance of the sensor. In this technology, a metal layer for holding down-spinning was sandwiched to allow spin-dependent scattering to occur in the magnetic layer.
A so-called synthetic pinned layer can be employed for the pinned layer of a CIP-GMR sensor. The synthetic pinned layer is a pinned layer of a stacked structure having a first pinned layer of a magnetic layer, a non-magnetic metal layer, and a second pinned layer of a magnetic layer stacked in this order with the first and second pinned layers being antiferromagnetically coupled. Namely, the entire SV sensor has a construction stacked in the order of substrate layer/antiferromagnetic layer/first pinned layer/first non-magnetic spacer layer/second pinned layer/second non-magnetic spacer layer/free layer/cap layer. In the synthetic pinned layer, because the directions of magnetization of the first and second pinned layers is antiparallel, the magnetization of the pinned layers is suppressed and stabilized. Furthermore, the synthetic pinned layer, in use for a read sensor of a head, can obviate a offset of the bias point caused by the static magnetic field induced by the pinned layer. The application of the synthetic pinned layer to a CPP-GMR sensor has been studied.
When a synthetic pinned layer is applied to a CPP-GMR sensor, the change in the resistance depends on the relative angle of magnetization of the second pinned layer with respect to the free layer. In the CPP structure, however, since the current flows through all the layers, the change in magnetoresistance is caused between the first pinned layer and the free layer, depending on the relative angle of magnetization. As described above, since the directions of magnetization of the second and first pinned layers are antiparallel, the relative directions of magnetization of the two pinned layers with respect to the free layer should also be opposite to each other. For this reason, the change in magnetoresistance that depends on the relative angle of magnetization between the first pinned layer and the free layer will work to cancel the change in magnetoresistance that depends on the relative angle of magnetization between the second pinned layer and the free layer.
In order to overcome the above-described adverse effect, the first pinned layer might be thinner to decrease the change in magnetoresistance caused by the bulk scattering of the first pinned layer and also to decrease the contribution of the resistance of the first pinned layer to the overall resistance of the sensor, while the second pinned layer thicker to increase the change in magnetoresistance caused by the bulk scattering of the second pinned layer and also to increase the contribution of the resistance of the second pinned layer to the overall resistance of the sensor. It is desirable, however, that the first and second pinned layers should have substantially the same amount of magnetization, and should be antiferromagnetically coupled in order to suppress the magnetization of the pinned layers. Accordingly, the configuration in which the thickness of the first and second pinned layers are significantly different is hard to realize. The pinned layer structure that meets such conflicted requirements has not been studied yet.